4 Answers
4

Equals is more idiomatic in American English, but either is acceptable.

To determine whether "three feet" is treated as singular or plural, try using it in other contexts. In American English, and I believe in UK English as well, you would say:

Three feet is pretty long for a garter snake

but:

Three feet are too many for a human being to have.

When expressing a distance, rather than describing the things your socks go on, "three feet" is treated as singular. Think of it as shorthand for "a distance of three feet" rather than "three of the items known as feet."

The trick with your phrase is that it can be interpreted in either way. Because it is a distance, you could think of it as:

A distance of three feet is equivalent to a distance of one yard.

in which case you use "equals." Or you could think of it as:

Three of the items known as feet are equivalent to one of the items known as yard.

in which case you use "equal."

Because either reading is possible, it's grammatically ambiguous. In my experience, though, "equals" is more common, following the way "three feet" would be treated in other sentences.

In "A distance of three feet is", "distance" is the subject and is singular, in "Three of the items known as feet are", the "items" are the subject and in plural. There is no ambiguity there.
–
msamApr 25 '14 at 14:31

3

And in "Three feet equal(s) one yard," "three feet" is the subject, and is neither unambiguously singular (since in this context it might be referring to three units of one foot) or plural (since we know it is treated as singular in most other contexts).
–
chapkaApr 25 '14 at 14:34

"Three feet are equal to a yard" would mean, "This foot is equal to a yard, this other foot is equal to a yard and so is this third foot. Other feet are not equal to a yard." That doesn't seem right.
–
David RicherbyApr 25 '14 at 15:42

So it would make perfect sense to use the word equals in speech and if you did it would be understood pretty much instantly. When people are speaking of a formula or equation they may use the word equals where an equals sign would be were they to write out their sentence. I don't see any issue with using the word equals so long as it is a translation of how the equation would have been written.

Using the word "is", on the other hand, makes sense for comparing items that are not measured.

For instance:

A banana is a fruit

or

Bob is a ninja

So the answer is both are fine to use. However, usage should depend on how technical what you are saying is.